I currently work full time and pay PAYE tax through my employer. I'm considering starting a little side project that involves offering a service through a white label 3rd party email provider. The aim is to set up email campaigns for clients (for free) then when they send the email I take a cut of the sending fee. I will never invoice the client for anything and my commission is paid directly by the 3rd party into a paypal account. Would I still need to set up as a sole trader or something to do this or can I just run it in the background? Would I owe tax on the commission? This is only intended to earn a bit of 'pocket money' i.e £200 - £300 per month ideally. Any advice would be greatly received.